The present invention relates to a particular type of a Thin Film Inorganic Light Emitting Diode.
At present several thin film electroluminescent devices (ELDs) exist or are still in development. They are all characterized by one (or more) electroluminescent active layer(s) sandwiched between two electrodes. Optionally a dielectric layer may also occur. The substrate can be plastic or glass.
The thin film ELDs can be subdivided into the organic and the inorganic based ELDs. The organic based thin film ELDs(1) are further subdivided into Organic molecular or Oligomeric Light Emitting Diodes (OLEDs) and Polymer Light Emitting Diodes (PLEDs). The inorganic ELDs on the other hand can be further subdivided into the High Voltage Alternating Current (HV-AC) ELDs and the Low Voltage Direct Current (LV-DC) ELDs. Amongst the HV-AC ELDs, one can distinguish Thin Film ElectroLuminescent Devices (TFEL devices or TFELDs) and Powder ELDs (PEL Devices or PELDs). Amongst LV-DC ELDs one can distinguish Powder ELDs (DC-PEL Devices or DC-PELDs) and thin film DC-ELDs, hereinafter called Inorganic Light Emitting Diodes (ILEDs).
The basic construction of organic ELDs (PLED and OLED) comprises following layer arrangement: a transparent substrate (glass or flexible plastic), a transparent conductor, e.g. Indium Tin Oxide (ITO), a hole transporting layer, a luminescent layer, and a second electrode, e.g. a Ca, Mg/Ag or Al/Li electrode. For OLEDs the hole transporting layer and the luminescent layer are 10-50 nm thick and applied by vacuum deposition; for PLEDs the hole transporting layer is 40 nm thick and the luminescent layer is 100 nm and applied by spin coating. Between both electrodes a direct voltage of 5-10 V is applied.
For OLEDs the hole transporting layer and electroluminescent layer consist of low molecular organic compounds, including oligomers. E.g. N,Nxe2x80x2-diphenyl-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (TPD) can be used as hole transporter, and as electroluminescent compounds the aluminium (III) 8-hydroxyquinoline complex (Alq3), polyaromatics (anthracene derivatives, perylene derivatives and stilbene derivatives) and polyhetero-aromatics (oxazoles, oxadiazoles, thiazoles etc.) can be used. The main advantages of these low molecular OLEDs include high fluorescent quantum yield, high purification and crystal growth capability and a wide selection of material design. Crystallisation of initially amorphous samples, production of exciplexes with other materials, and often high chemical reactivity create serious problems in their application to stable EL systems. Moreover, the layers are applied by using vacuum-vapour deposition techniques, which are often cumbersome and expensive. However, recently monochromic displays based on this principle are launched on the market by Pioneer(3).
On the other hand the electroluminescent compounds that can be used in PLEDs are polymers like the non-conjugated polyvinylcarbazole derivatives (PVK) or the conjugated polymers like poly(p-phenylene vinylenes) (PPV), poly(3-alkylothiophene, poly(p-phenylene ethynylenes) etc. These high molecular weight materials allow for the easy preparation of thin films by casting, and show a high resistance to crystallization. The difficulties in purifiation procedures (and hence reproducability) and, mostly, a low fluorescent quantum yield, in addition to a complex structure and high sensitivity towards oxygen and moisture are severe drawbacks in their application to organic ELDs.
As mentioned before, today, two kinds of HV-AC inorganic based ELDs exist(2): PELDs and TFELDs. PEL Devices are used mainly for backlighting and low information content displays and can be manufactured on flexible substrates. TFEL devices are used for high information content matrix displays and can only be produced on glass substrates, due to the high processing temperature needed. Their basic structure is essentially the same. A PELD consists of a transparent substrate, a transparent conductor (ITO), a luminescent layer of doped ZnS (50 xcexcm), an insulator/reflector layer (50 xcexcm), and a top electrode, e.g. Ag-paste. Between both electrodes an alternating voltage of 110 V and 400 Hz is applied. A TFEL device consists of a transparent substrate, a transparent conductor, and a luminescent phosphor layer (0.6 xcexcm) between two moisture protective insulator layers (0.6 xcexcm), and a top electrode, e.g. evaporated metal. Between both electrodes an alternating voltage of 200 V and 400 Hz is applied.
For both devices, doped ZnS is used as the active luminescent layer. The doping centers (e.g., Mn2+, Cu+) can be excited by the impact of injected electrons by means of a high electric field or by transfer of recombination energy towards an electroluminescent centre. These excited centers relax to the ground state by a luminescence process. Several mechanisms for the electroluminescence process have been proposed(4). The insulating layer for the PELD (screen printed BaTiO3xe2x88x92powder+binder) fulfils three functions: (1) it prevents catastrophic breakdowns, (2) due to the high dielectric constant, the electric field will predominantly be focussed over the ZnS layer and (3) it will serve as diffuse light reflector, which increases the light output. For the TFEL device the vacuum deposited Al2O3 prevents also catastrophic breakdowns and it functions as tunnelling barrier for the electron injection. The structures are completed by two electrodes, one of which should be transparent.
For the PEL device, the ZnS layer comprises ZnS powder doped with copper ions and possibly other elements like chlorine and manganese mixed in an organic binder with a high dielectric constant like polyvinylidene fluoride or some kind of cyanoresin. The powders are prepared by high temperature sintering processes ( greater than 700xc2x0 C.) in order to allow diffusion of the dopants into the ZnS crystal matrix and integration of CuxS-rich needles in- and outside the ZnS latice after cooling. For the HV-AC PELDs, the CuxS-needles at the surface are chemically removed by etching with cyanide ions. This to ensure electrical isolating particles. After that the particles are provided with a moisture protective layer like Al2O3 or NC (carbon-nitride) in order to increase the lifetime of the phosphor. The particle size of these powders varies between 2-20 xcexcm. The dispersion of this powders can be used in a screen printing process in order to apply them in an electroluminescent layer with a thickness of 50-100 xcexcm.
Upon applying an electric field on the phosphor particles, charges (e.g. electrons) are generated inside the particles at stacking faults in the ZnS latice and/or in the CuxS needles. These charges can move according to the direction of the applied field (AC). By this way recombination processes can occur whereby the recombination energy can be transferred to an electroluminescent centre (e.g. Cu+ or Mn2+). Also direct impact excitation of the luminescence centre can occur if the kinetic energy of the electron is high enough (ca 4-5 eV). These processes can give rise to electroluminescence.
Electroluminescence occurs normally at field strengths in the order of 1-2 MV/cm. For a layer thickness of 100 xcexcm, applying 110 V results in a mean value of the field strength of 50 kV/cm. The CuxS needles at the interior of the ZnS-particles increase locally the electric field strength by a factor of 50 to 100 resulting in field strength values of up to 1 MV/cm, thereby making electroluminescence possible. The thick layer causes a slow luminance-voltage response, making PEL devices unsuitable for display applications.
General drawbacks of these PELDs are price (large amounts of products needed (about 150 g phosphor per square meter), the expensive BaTiO3 layer, the high driving voltages (around 110 V) and their inherent instability. The latter is caused by the slow decrease in number of crystal stacking faults and CuXS needles by applying an electric field (recrystallisation). The less stacking faults or CuxS needles, the less charges (electrons) can be generated and hence a decrease in light output can be expected.
For the TFEL device, the doped ZnS layer is preferably deposited by using the Atomic Layer Epitaxy technology (Planar International), which also requires a high temperature sintering step, thereby limiting the substrates to glass substrates. The formed doped ZnS-layer is crystalline. This layer is sandwiched between two thin isolating Al2O3 layers. Upon applying a high voltage, typically between 150-200 V, tunneling of electrons occurs through these Al2O3 layers. Due to the high crystallinity of the ZnS layer, the electrons can travel freely under the influence of the applied electric field in this doped ZnS layer, thereby accumulating kinetic energy. At accumulation values of about 4-5 eV, impact excitation of the dopant can occur resulting in electroluminescence. The thin electroluminescent layer (1-1.5 xcexcm) shows a treshhold behaviour of the luminance-voltage curve, making this TFEL device suitable for display applications.
General drawbacks of this system are the slow and expensive production process and the necessity of complete protection against ambient humidity.
Low voltage DC PEL Devices were developed by Vecht et al, as described by Chadha(5). They used the same phosphor particles as for the AC-PELDs, but the conductive CuxS-needles at the surface are not removed. The general construction consists of a transparent substrate, a transparent conductor (ITO), a doped ZnS phosphor layer (20 xcexcm), and a top electrode (evaporated Alu).
The phosphor layer is applied by means of the doctor blade technique or screen printing on an ITO conducting layer. Subsequently an Alu electrode is evaporated. Upon applying a direct current voltage of several volts (ITO positive), the p-conductive CuxS starts moving towards the Alu electrode, thereby creating an insulating region (about 1 xcexcm in thickness) next to the ITO layer within one minute or so. This results in a current drop which is associated with the onset of light emission. This process has been called the forming process. In this formed thin high resistive phosphor layer, high electric fields occur and electroluminescence is already possible at low voltages (typically between 10 and 30 V).
Major drawbacks of this system are the irreproducability of the forming process and the necessity of complete encapsulation against moisture. Also the preparation of the phosphor particles requires high sintering temperatures. No commercial devices based on this principle are on the market today.
Recently, several research groups reported photo-(7-13) and electroluminescence(14-18) from inorganic semiconducting nano particles.
Colvin et al.(14) reported on the electroluminescence of CdSe nano-particles stabilized by hexane dithiol. They demonstrated EL for two devices comprising a spincoated double layer of CdSe and PPV on ITO and covered it with an evaporated Mg electrode. Depending on the voltage they observed emission from the CdSe (lower voltages) or from the PPV (higher voltages).
Electroluminescence of CdSe quantum-dot/polymer composites was also reported by Dabbousi et al.(15). They spincoated on ITO one single layer of CdSe nano-particles stabilized with trioctylphosphine oxide and mixed with a polymeric hole transporter (PVK) and an electron transport species (an oxadiazole derivative of PVK, t-Bu-PBD). An aluminum electrode was subsequently evaporated. The system showed electroluminescence in reverse bias, and depending on the applied voltage the emission spectrum of the CdSe quantumdots or PVK was observed.
Gao et al.(16) reported on the electroluminescence of self-assembled films of PPV and CdSe nano-particles. They could observe electroluminescence from the CdSe particles and/or from the PPV, depending on the applied voltage.
These examples demonstrate the possible use of inorganic nano-particles with semiconductor properties as Light Emitting Diodes (ILED), in analogy with the OLEDs. However, the use of Cd- or Se-compounds can not be recommended due to environmental problems that can be expected.
Huang et al.(17) reported the photo- and electroluminescence of a single layer of ZnS:Cu nanocrystals spincoated on a ITO substrate and evaporated with an aluminum electrode. ZnS and CuxS are much more environmental friendly compared to CdSe. Also there was no need for organic hole or electron transporters, which can cause stability problems as is known in the organic PELDs. The drawback of their system lies in the fact that the synthesis of the ZnS:Cu particles is quite cumbersome and results in low yields. Polystyrene sulphonic acid is used as polyelectrolyte on which Zn and Cu ions are attached. Subsequently this polyelectrolyte is solved in dimethylformamide and reacted with H2S. By this way ZnS:CxS particles are formed.
Que et al.(18) reported photo- and electroluminescence from a copper doped ZnS nanocrystals/polymer composite. The synthesis of the nano-particles was carried out by using the inverse microemulsion method. After washing and drying the ZnS:Cu powder was redispersed in MEK with PMMA as a binder and spincoated on ITO and evaporated with an aluminum electrode. Green electroluminescence could be observed in both bias directions at 5 V. The drawback of the fabrication of this device is the low concentrations of the ZnS:Cu dispersion that can be obtained (ca 10xe2x88x923 M). Further it needs a well defined two phase system (soap/water). Also a drawback could be the solvent based spincoating dispersion.
References
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The present invention extends the teachings on thin film electroluminescent devices.
It is the object of the present invention to provide an easy and economical method for the manufacturing of a Thin Film Inorganic Light Emitting Diode device capable of emitting light under the influence of direct current.
The above mentioned objects are realized by providing a method for manufacturing a Thin Film Inorganic Light Emitting Diode device, said method comprising the following steps, in order,
(1) preparing a nanoparticle dispersion of ZnS doped with a luminescent centre by precipitation from appropriate aqueous solutions comprising zinc ions, sulfide ions and dopant ions,
(2) washing said dispersion of doped ZnS to remove non-precipitated ions, either,
(3) mixing said washed dispersion of doped ZnS (n-type semiconductor) with a water-compatible p-type semiconductive polymer,
(4) coating said mixture, optionally after admixture with a binder, onto a first conductive electrode,
(5) applying on top of said coated layer resulting from step (4) a second conductive electrode, with the proviso that at least one of said first and second electrode is transparent, or,
(3xe2x80x2) coating on top of a first conductive electrode a double layer pack comprising, in either order,
(3xe2x80x2a) a layer containing a water-compatible p-type semiconductive polymer, and,
(3xe2x80x2b) a layer containing said washed dispersion of doped ZnS, optionally admixed with a binder,
(4xe2x80x2) applying on top of said coated layer pack resulting from step (3xe2x80x2) a second conductive electrode, with the proviso that at least one of said first and second electrode is transparent.
In a preferred embodiment the water-compatible p-type semiconductive polymer is a polythiophene/polymeric polyanion complex, most preferably a poly(3,4-ethylenedioxythiophene)/polystyrene sulphonate complex.
Preferred dopants for ZnS are manganese and copper ions.